JP2000-78783A published by the Japan patent office in 2000 discloses a motor/generator provided with a rotor comprising a permanent magnet. This permanent magnet has a surface formed as a bulging arc facing the center of the rotor. The area of the permanent magnet in contact with the rotor is made to increase, and local concentration of magnetic flux transmitted to the adjacent permanent magnet is mitigated. Thus, magnetic loss can be reduced and motor efficiency can be improved.